Nickel-manganese composite hydroxide, method for producing the same, positive electrode active material for nonaqueous electrolyte secondary battery, method for producing the same, and nonaqueous electrolyte secondary battery

ABSTRACT

Provided are a positive electrode active material with which a nonaqueous electrolyte secondary battery having a high energy density can be obtained, a nickel-manganese composite hydroxide suitable as a precursor of the positive electrode active material, and production methods capable of easily producing these in an industrial scale. Provided is a nickel-manganese composite hydroxide represented by General Formula (1): NixMnyMz(OH)2+α and containing a secondary particle formed of a plurality of flocculated primary particles. The nickel-manganese composite hydroxide has a half width of a diffraction peak of a (001) plane obtained by X-ray diffraction measurement of at least 0.10° and up to 0.40° and has a degree of sparsity/density represented by [(void area within secondary particle/cross section of secondary particle)×100](%) of at least 0.5% and up to 10%. Also provided is a production method of the nickel-manganese composite hydroxide.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 17/511,062filed on Oct. 26, 2021 which is a divisional of U.S. application Ser.No. 16/320,601 Filed on Jan. 25, 2019, which claims priority and thebenefit of the International Application No. PCT/JP2017/027538 filedJul. 28, 2017, which claims priority and the benefit of Japaneseapplication No. 2016-150505 filed on Jul. 29, 2016, the entiredisclosures of which are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a nickel-manganese composite hydroxide,a method for producing the same, a positive electrode active materialfor a nonaqueous electrolyte secondary battery, a method for producingthe same, and a nonaqueous electrolyte secondary battery.

BACKGROUND ART

In recent years, with the proliferation of portable electronic equipmentsuch as cellular phones and notebook personal computers, development ofa nonaqueous electrolyte secondary battery with reduced size and weighthaving high energy density is intensely demanded. A representativeexample of such a nonaqueous electrolyte secondary battery is a lithiumion secondary battery. For a negative electrode active material of thelithium ion secondary battery, lithium metal, lithium alloys, metaloxides, carbon, and the like are being used. These materials arematerials that can de-insert and insert lithium.

Currently, research and development of lithium ion secondary batteriesare being energetically conducted. Among them, lithium ion secondarybatteries using lithium-transition metal composite oxides, especially alithium-cobalt composite oxide (LiCoO₂), which is relatively easilysynthesized, for a positive electrode active material can obtain as highvoltage as 4 V class and are thus expected as batteries having highenergy density and are in practical use. Also being developed are alithium-nickel composite oxide (LiNiO₂), alithium-nickel-cobalt-manganese composite oxide(LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂), and the like using nickel, which islower in price than cobalt, as the positive electrode active material.Among them, the lithium-nickel-cobalt-manganese composite oxide attractsattention because of its excellent balance among battery capacity,output characteristics, durability, costs, and the like. However, it isinferior to lithium-nickel composite oxide-based ones in capacity, and asufficient output characteristic and improvement of its battery capacity(energy density) are required.

Various developments have been made in response to the requirements toimprove battery capacity in the positive electrode active material.Patent Literature 1 presents a positive electrode active material for anonaqueous electrolyte secondary battery having an average particlediameter of 2 to 8 μm and [(D90−D10)/an average particle diameter] as anindicator indicating a spread of particle size distribution of up to0.60 in order to improve cycle characteristics and achieve high output,for example. Such an active material causes an electrochemical reactionto uniformly occur and has the advantages of high capacity and longlife, but on the other hand, it is low in fillability of the positiveelectrode active material and is thus not high in volume energy density.

In addition, for example, Patent Literature 2 proposes a method forproducing a positive electrode active material for a lithium ionbattery, in which hydroxide raw material powder is crushed, and thenslurry including the crushed raw material powder having a specificparticle size distribution is prepared, and after nearly sphericallygranulated powder is prepared by using this slurry and then mixed with alithium compound, the granulated powder is caused to react with thelithium compound by firing. It is reported that with this method thepositive electrode active material having a desired void rate and a highopen pore ratio thereby providing excellent battery characteristics canbe obtained. However, this method requires a process in which after theobtained hydroxide is crushed, it is granulated again so as to obtain aprecursor; and thus, in this method there is a problem in productivity.In addition, when the open pore ratio is increased, even though thebattery characteristics can be improved, there is a problem in that avolume energy density decreases.

Furthermore, Patent Literature 3 presents a nickel-cobalt-manganesecomposite hydroxide obtained by being precipitated by holding an aqueoussolution containing a nickel salt, a cobalt salt, and a manganese saltat at least pH 10 and up to pH 13 in an atmosphere of a mixture gas ofan inert gas and an oxygen gas with a volume ratio relative to the inertgas of at least 0.5% and up to 3.0% and a positive electrode activematerial for a nonaqueous electrolyte secondary battery obtained byfiring a mixture of the composite hydroxide and a lithium compound, forexample. It is said that with this, the tap density and the bulk densityof the nickel-cobalt-manganese composite hydroxide can be improved, thepositive electrode active material and a precursor thereof can beincreased in density, and the capacity of a nonaqueous electrolytesecondary battery can be further improved. However, although batterycapacity is studied, other battery characteristics have not been fullystudied.

Patent Literature 4 proposes a positive electrode active material for anonaqueous secondary battery in which the material has the averageparticle diameter of more than 8 μm and up to 16 μm and includes a shellportion with [(D90−D10)/average particle diameter] that is an indicatorto represent a spread of the particle size distribution of up to 0.60and a hollow portion inside the shell portion. It is reported that thispositive electrode active material has a uniform particle sizedistribution and a high fillability, and that this can decrease apositive electrode resistance value. However, there is a problem in thehollow particle that the fillability decreases even though a high outputcharacteristic can be obtained. Here, the shape of the primary particleof the hydroxide is controlled by switching the atmosphere during thetime of crystallization, which requires the time for switching; andthus, there is also a problem of lowered productivity.

Patent Literature 5 proposes a method for producing a nickel-manganesecomposite hydroxide particle, in which a raw material aqueous solutioncontaining at least nickel and manganese, an aqueous solution includingan ammonium-ion-providing body, and an alkali solution are fed into areaction vessel, and they are mixed to form a reaction aqueous solution;and during the time when the nickel-manganese composite hydroxideparticles are crystallized out, an oxygen concentration inside thereaction vessel is made up to 3.0% by volume, a temperature of thereaction aqueous solution is controlled in the range of 35 to 60° C.,and a nickel concentration is controlled at at least 1,000 mg/L. It isreported that with this method, the circularity of the nickel-manganesecomposite hydroxide particle can be enhanced, so that the fillability ofthe positive electrode active material including the nickel-manganesecomposite hydroxide particle serving as a precursor can be enhanced.However, this proposal pays an attention only to the fillability that isenhanced by sphericity of the particle; and thus, there still remains aroom for the study about the volume energy density.

CITATION LIST Patent Literatures

-   [Patent Literature 1] Japanese Unexamined Patent Application    Publication No. 2011-116580-   [Patent Literature 2] Japanese Unexamined Patent Application    Publication No. 2015-76397-   [Patent Literature 3] Japanese Unexamined Patent Application    Publication No. 2013-144625-   [Patent Literature 4] International Publication No. WO 2012/169274-   [Patent Literature 5] International Publication No. WO 2015/115547

SUMMARY OF INVENTION Technical Problems

As described above, a further enhanced energy density is requested tothe nonaqueous electrolyte secondary battery. Therefore, in order torespond to such a request, various positive electrode active materialshave been proposed. However, as of today, the positive electrode activematerial that satisfies the requirements of a high volume energy densityand a sufficient output characteristic by appropriately controlling thefillability and the battery capacity has not been developed yet. It hasbeen known that the fillability and the battery capacity of the positiveelectrode active material can be improved, for example, by compatiblysatisfying, in a high level, the tap density and the specific surfacearea of a composite hydroxide, which is a precursor of the positiveelectrode active material; and thus, a method for producing thecomposite hydroxide (precursor) is also investigated extensively.However, as of today, the method for producing, in an industrial scale,the composite hydroxide (precursor) capable of sufficiently enhancingthe performance of a lithium ion secondary battery has not beendeveloped yet. Accordingly, there is a demand to develop the methods forproducing a positive electrode active material that has a high volumeenergy density and a sufficient output characteristic and a compositehydroxide that is a precursor thereof at low cost and on a large scale.

In view of the problems as described above, the present inventionintends to provide: a positive electrode active material with which anonaqueous electrolyte secondary battery having a high energy densityand a sufficient output characteristic as a secondary battery can beproduced; and a nickel-manganese composite hydroxide that is suitable asa precursor thereof. In addition, the present invention intends toprovide a method for easily producing a nickel-manganese compositehydroxide and a method for producing a positive electrode activematerial for a nonaqueous electrolyte secondary battery by using thisnickel-manganese composite hydroxide, both in an industrial scale.

Solution to Problems

A first aspect of the present invention provides a nickel-manganesecomposite hydroxide, in which the nickel-manganese composite hydroxideis represented by General Formula (1): Ni_(x)Mn_(y)M_(z)(OH)_(2+α) (inFormula (1), M is at least one element selected from Co, Ti, V, Cr, Zr,Nb, Mo, Hf, Ta, Fe, and W; and x, y, z, and a satisfy 0.1≤x≤0.9,0.05≤y≤0.8, 0≤z≤0.8, x+y+z=1.0, and 0≤α≤0.4) and contains a secondaryparticle formed of a plurality of flocculated primary particles. Thenickel-manganese composite hydroxide has a half width of a diffractionpeak of a (001) plane obtained by X-ray diffraction measurement of atleast 0.10° and up to 0.40° and has a degree of sparsity/densityrepresented by [(void area within secondary particle/cross section ofsecondary particle)×100](%) of at least 0.5% and up to 10%.

In addition, in the nickel-manganese composite hydroxide, it ispreferable that a pore volume that is measured by a nitrogen adsorptionmethod be at least 0.01 cm³/g and up to 0.04 cm³/g. In addition, in thenickel-manganese composite hydroxide, it is preferable that[(D90−D10)/average particle diameter] that is an indicator to representa spread of particle size distribution be at least 0.7, and avolume-average particle diameter MV be at least 5 μm and up to 20 μm. Inaddition, in the nickel-manganese composite hydroxide, it is preferablethat a specific surface area be at least 5 m²/g and up to 15 m²/g. Inaddition, in the nickel-manganese composite hydroxide, it is preferablethat a tap density be at least 1.8 g/cm³ and up to 2.5 g/cm³.

A second aspect of the present invention provides a nickel-manganesecomposite hydroxide represented by General Formula (1): NixMnyMz(OH)2+α(in Formula (1), M is at least one element selected from Co, Ti, V, Cr,Zr, Nb, Mo, Hf, Ta, Fe, and W; x satisfies 0.1≤x≤0.9, y satisfies0.05≤y≤0.8, z satisfies 0≤z≤0.8, and x+y+z=1.0; and a satisfies 0≤α≤0.4)and containing a secondary particle formed of a plurality of flocculatedprimary particles, the method including a crystallization process ofgenerating a nickel-manganese composite hydroxide by neutralizing a saltcontaining at least nickel and a salt containing at least manganese in areaction aqueous solution, in the crystallization process, a dissolvedoxygen concentration in the reaction aqueous solution being adjusted tofall within a range of at least 0.2 mg/L and up to 4.6 mg/L, and adissolved nickel concentration in the reaction aqueous solution beingadjusted to fall within a range of at least 700 mg/L and up to 1,500mg/L.

In addition, in the crystallization process, it is preferable that astirring power loaded to the reaction aqueous solution be adjusted tofall within a range of at least 3 kW/m³ and up to 15 kW/m³. In addition,in the crystallization process, it is preferable that a temperature ofthe reaction aqueous solution be adjusted to fall within a range of atleast 35° C. and up to 60° C. In addition, in the crystallizationprocess, it is preferable that a pH value measured based on atemperature of the reaction aqueous solution at 25° C. be adjusted tofall within a range of at least 10.0 and up to 13.0. In addition, thecrystallization process preferably includes continuously adding a mixedaqueous solution including nickel and manganese into a reaction vesseland overflowing slurry including nickel-manganese composite hydroxideparticles formed by neutralization to recover the secondary particle.

A third aspect of the present invention provides a positive electrodeactive material for a nonaqueous electrolyte secondary battery, thepositive electrode active material including a lithium-nickel-manganesecomposite oxide represented by General Formula (2): Li1+tNixMnyMzO2+β(in Formula (2), M is at least one additional element selected from Co,Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Fe, and W; t satisfies −0.05≤t≤0.5, xsatisfies 0.1≤x≤0.9, y satisfies 0.05≤y≤0.8, z satisfies 0≤z≤0.8, andx+y+z=1.0; and β satisfies 0≤β≤0.5) and containing a secondary particleformed of flocculated primary particles, the positive electrode activematerial for a nonaqueous electrolyte secondary battery having a degreeof sparsity/density represented by [(void area within secondaryparticle/cross section of secondary particle)×100](%) of at least 0.5%and up to 12% and having a DBP absorption amount measured in compliancewith JIS K6217-4 of at least 12 cm^(3/100) g and up to 20 cm^(3/100) g.

In addition, it is preferable that a tap density is at least 2.0 g/cm³and up to 2.7 g/cm³. In addition, it is preferable that a ratioI(003)/I(104) of a diffraction peak intensity I(003) of a 003 plane to apeak intensity I(104) of a 104 plane, obtained by X-ray diffractionmeasurement, be at least 1.7. In addition, when an arbitrary radialdirection from a center of a cross section of the secondary particletoward an outside thereof is regarded as an x-axis direction and adirection perpendicular to the x-axis direction is regarded as a y-axisdirection, it is preferable that an orientation rate of a crystal abplane measured by an electron backscatter diffraction method be at least55% in each of the x-axis direction and the y-axis direction.

A fourth aspect of the present invention provides a method for producinga positive electrode active material for a nonaqueous electrolytesecondary battery represented by General Formula (2)Li_(1+t)Ni_(x)Mn_(y)M_(z)O_(2+β) (in Formula (2), M is at least oneadditional element selected from Co, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Fe,and W; t satisfies −0.05≤t≤0.5, x satisfies 0.1≤x≤0.9, y satisfies0.05≤y≤0.8, z satisfies 0≤z≤0.8, and x+y+z=1.0; and β satisfies 0≤β≤0.5)and containing a secondary particle formed of flocculated primaryparticles, the method including a process of obtaining a mixture bymixing the nickel-manganese composite hydroxide and a lithium compoundtogether and a process of obtaining a lithium-nickel-manganese compositeoxide by firing the mixture.

It is preferable that the nickel-manganese composite hydroxide beobtained by the above-described production method of thenickel-manganese composite hydroxide.

A fifth aspect of the present invention provides a nonaqueouselectrolyte secondary battery containing the positive electrode activematerial for a nonaqueous electrolyte secondary battery in a positiveelectrode.

Advantageous Effects of the Invention

With the positive electrode active material of the present invention, anonaqueous electrolyte secondary battery having a high energy densityand a sufficient output characteristic as a secondary battery can beobtained. In addition, the nickel-manganese composite hydroxide of thepresent invention has an excellent fillability, so that this can besuitably used as the precursor of the positive electrode activematerial. In addition, with the methods of the present invention toproduce the nickel-manganese composite hydroxide and the positiveelectrode active material, these can be produced easily in an industrialscale; and thus, it can be said that industrial values thereof are veryhigh.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing that illustrates one example of thenickel-manganese composite hydroxide of the present embodiment.

FIG. 2 is a drawing that illustrates one example of the productionmethod of the nickel-manganese composite hydroxide of the presentembodiment.

FIG. 3 includes schematic drawings that illustrates one example of thelithium-nickel-manganese composite oxide of the present embodiment.

FIG. 4 is a drawing that illustrates one example of the productionmethod of the lithium-nickel-manganese composite oxide of the presentembodiment.

FIG. 5 includes pictures that illustrate one example of outerappearances and cross sections of the nickel-manganese compositehydroxide of the present embodiment.

FIG. 6 includes pictures that illustrate outer appearances and crosssections of the positive electrode active material of the presentembodiment.

FIG. 7 includes explanatory drawings of the evaluation method of thecrystal orientation of the positive electrode active material by usingthe electron backscatter diffraction method (EBSD).

FIG. 8 is a schematic cross-sectional view of a coin-type battery usedfor evaluation of the battery characteristics.

FIG. 9 is a drawing that illustrates one example of the Nyquist plotobtained by the alternate current impedance method.

FIG. 10 is a schematic explanatory drawing that illustrates theequivalent circuit used for the analysis of the impedance evaluation.

DESCRIPTION OF EMBODIMENTS

The following describes details of a nickel-manganese compositehydroxide, a method for producing the same, a positive electrode activematerial for a nonaqueous electrolyte secondary battery, and a methodfor producing the same of the present embodiment with reference to theaccompanying drawings. In the drawings, to make components easier tounderstand, they are illustrated with a part emphasized or with a partsimplified, and actual structures or shapes, a reduced scale, and thelike may be different.

(1) Nickel-Manganese Composite Hydroxide

FIG. 1 is a schematic diagram of an exemplary nickel-manganese compositehydroxide of the present embodiment. As illustrated in FIG. 1 , thisnickel-manganese composite hydroxide 1 (hereinafter, also referred to asa “composite hydroxide 1”) is containing a secondary particle 3 formedof a plurality of flocculated primary particles 2. The secondaryparticle 3 has a void 4 among the primary particles 2. Although thecomposite hydroxide 1 mainly includes the secondary particle 3 formed ofthe flocculated primary particles 2, it may contain a small number ofprimary particles 2 such as a primary particle 2 that has not beenflocculated as the secondary particle 3 and a primary particle 2 thathas fallen from the secondary particle 3 after being flocculated.

In the composite hydroxide 1 of the present embodiment, as will bedescribed later, during the crystallization reaction, the dissolvedoxygen concentration and the dissolved nickel concentration in thereaction aqueous solution, and also preferably the stirring powertherein are adjusted so as to control the crystallinity and the degreeof sparsity/density in specific respective ranges; and thus, thenonaqueous electrolyte secondary battery (hereinafter, this is alsoreferred to as “secondary battery”) including the positive electrodeactive material for a nonaqueous electrolyte secondary battery(hereinafter, this material is also referred to as “positive electrodeactive material”) using this composite hydroxide 1 as the precursorthereof can have a very high energy density as well as a sufficientoutput characteristic as the secondary battery.

The composite hydroxide 1 is represented by General Formula (1):Ni_(x)Mn_(y)M_(z)(OH)_(2+α). In Formula (1), M is at least oneadditional element selected from. Co, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Fe,and W; x satisfies 0.1≤x≤0.9, y satisfies 0.05≤y≤0.8, z satisfies0≤z≤0.8, a satisfies 0≤α≤0.4, and x+y+z=1.0. In Formula (1), a is acoefficient that changes in accordance with the valence number of themetal elements contained in the composite hydroxide 1.

In Formula (1), when y indicating the content of Mn in the compositehydroxide 1 is within the above range, the morphology of the primaryparticles 2 can be adjusted in accordance with the dissolved oxygenconcentration in the reaction aqueous solution in the crystallizationprocess, whereby the degree of sparsity/density can be controlled tofall within a desired range. In view of controlling the degree ofsparsity/density more precisely, y preferably satisfies 0.1≤y≤0.8. Whenthe value of y is at least 0.1, the degree of sparsity/density of thesecondary particle 3 can be controlled at a lower dissolved oxygenconcentration, and thus excessive oxidation of transition metals can beprevented. When in Formula (1) z indicating the content of M is greaterthan 0, requirements for various battery characteristics can besatisfied. M containing Co gives more excellent battery capacity andoutput characteristics, for example. When M is Co, z preferablysatisfies 0.1≤z≤0.4.

The half width of the diffraction peak of the (001) plane obtained fromthe XRD measurement of the composite hydroxide 1 is at least 0.10° andup to 0.40°, and preferably at least 0.20° and less than 0.40°. The halfwidth of the (001) plane is a factor to influence the size andorientation of the crystallite that constitutes the composite hydroxide1. When the half width of the (001) plane is within the range describedabove, the primary particles have a high crystallinity, and in addition,an appropriate orientation; and thus, the secondary particle thereof canbe made to have a dense structure so that the positive electrode activematerial using this hydroxide can have a high energy density and canretain a sufficient output characteristic as the secondary battery. Whenthe half width of the (001) plane is less than 0.10°, the crystallinitythereof is too high that the reactivity with a Li compound uponobtaining the positive electrode active material is decreased; and thus,the positive electrode active material having desired characteristicscannot be obtained. On the other hand, when the half width of the (001)plane is more than 0.40°, the positive electrode active materialobtained from the composite hydroxide 1 is prone to be a sparse particle(secondary particle 3 having a high degree of sparsity/density), or theorientation of the primary particle of the positive electrode activematerial may decrease. Note that the diffraction peak of the (001) planeappears near 2θ=19° (2θ=19±1°).

In the composite hydroxide 1, the degree of sparsity/density measuredfrom the image of the cross section of the secondary particle thereof bymeans of a scanning electron microscope (SEM) is at least 0.5% and up to10%, and preferably at least 0.5% and up to 8%. When the degree ofsparsity/density is within the range described above, the positiveelectrode active material having excellent battery capacity andfillability, as well as a further enhanced volume energy density and asufficient output characteristic can be obtained. On the other hand,when the degree of sparsity/density is less than 0.5%, penetration ofthe Li compound into the particle is insufficient during the time ofpreparing the positive electrode active material; and thus, thereactivity with the lithium compound may decrease. When the degree ofsparsity/density is more than 10%, the volume energy density maydecrease.

Here, “degree of sparsity/density” is the value obtained from the resultof the image analysis of the cross section of the particle of compositehydroxide 1 by using a scanning electron microscope (SEM), and this isthe value represented by [(area of the void 4 inside the secondaryparticle 3/area of the cross section of the secondary particle3)×100](%). For example, in the cross section of the composite hydroxide1 depicted in FIG. 1 , the degree of sparsity/density is the valuerepresented by [(area of the void 4)/(sum of the area of the crosssection of the primary particle 2 and the area of void 4)×100]. Namely,the higher the degree of sparsity/density is, the sparser is thestructure inside the secondary particle 3; and the lower the degree ofsparsity/density is, the denser is the structure inside the secondaryparticle 3. Here, as the degree of sparsity/density, an average degreeof sparsity/density can be used, in which this can be obtained in such away that the cross sections of 20 secondary particles 3 that are atleast 80% of the volume-average particle diameter (MV) are randomlyselected, and the degree of sparsity/density of each cross section ofthe secondary particles 3 is measured followed by averaging therespective values.

In the composite hydroxide 1, the pore volume that is measured by anitrogen adsorption method is preferably at least 0.01 cm³/g and up to0.04 cm³/g. When the pore volume is less than 0.01 mL/g, penetration ofthe Li compound into the particle during the time of obtaining thepositive electrode active material is insufficient, so that thereactivity with the lithium compound may decrease. When the pore volumeis within the range described above, an excellent fillability and asuitable output characteristic as the positive electrode active material10 can be obtained.

The particle diameter of the composite hydroxide 1 is not particularlylimited, so that any of the desired range thereof may be allowed. Whenthis is used for a precursor of the positive electrode active material,the volume average particle diameter MV is preferably at least 5 μm andup to 20 μm and more preferably at least 6 μm and up to 15 μm. When theaverage particle diameter is less than 5 μm, the fillability of theparticle of the composite hydroxide 1 significantly decreases, so thatit may be difficult to increase the battery capacity per volume whenthis is made into the positive electrode active material. On the otherhand, when the average particle is more than 20 μm, because the specificsurface area decreases, the reactivity with the lithium raw materialupon making the positive electrode active material decreases, so thatthe positive electrode active material having good batterycharacteristics may not be obtained. In the obtained positive electrodeactive material, the interface with the electrolyte solution decreasesthereby leading to an increase in the positive electrode resistance, sothat the output characteristic of the battery may deteriorate.

In the composite hydroxide 1, it is preferable that [(D90−D10)/averageparticle diameter] that is an indicator to represent a spread of theparticle size distribution be at least 0.7. When [(D90−D10)/averageparticle diameter] is less than 0.7, uniformity of the particle diameteris enhanced thereby tending to increase the charging and dischargingcapacities per mass (hereinafter, this is also referred to as “batterycapacity”); but the particle fillability may decrease thereby leading toa decrease in the volume average density. The [(D90−D10)/averageparticle diameter] may be adjusted within the range described above, forexample, by mixing the composite hydroxide 1 having different particlediameters, or by producing the composite hydroxide 1 by a continuouscrystallization method. Here, the upper limit of [(D90−D10)/averageparticle diameter] is not particularly limited, while in view ofsuppressing excessive contamination of fine particles or coarseparticles into the positive electrode active material, the upper limitthereof is preferably, for example, up to 1.2 and more preferably up to1.0.

In [(D90−D10)/the average particle diameter], D10 means a particlediameter at which, when the numbers of particles of the respectiveparticle diameters are accumulated from a smaller particle diameter, theaccumulated volume reaches 10% of the total volume of all the particles,and D90 means a particle diameter at which, when the numbers ofparticles are accumulated similarly, the accumulated volume reaches 90%of the total volume of all the particles. The average particle diameteris the volume-average particle diameter MV, which means an averageparticle diameter weighted in terms of volume. The volume-averageparticle diameter MV, and D90 and D10 can be measured using a laserdiffraction/scattering particle size analyzer.

The specific surface area of the composite hydroxide 1 is preferably inthe range of at least 2.5 m²/g and up to 50 m²/g and more preferably inthe range of at least 5 m²/g and up to 15 m²/g. When the specificsurface area is within the range described above, the positive electrodeactive material using the composite hydroxide 1 as the precursor canhave further enhanced battery characteristics and fillability. Thespecific surface area may be brought within the range described above byadjusting the degree of sparsity/density and the particle distributionincluding the average particle diameter MV of the composite hydroxide 1.

A tap density of the composite hydroxide 1 is preferably within a rangeof at least 1.8 g/cm³ and up to 2.5 g/cm³ and more preferably of atleast 1.9 g/cm³ and up to 2.5 g/cm³. When the tap density is within theabove range, the positive electrode active material using the compositehydroxide 1 as a precursor is more excellent in fillability, achievingimprovement in battery capacity. The tap density can be made within theabove range by adjusting the particle size distribution including theaverage particle diameter MV or the degree of sparsity/density of thecomposite hydroxide 1.

(2) Method for Producing Nickel-Manganese Composite Hydroxide

FIG. 2 is a diagram of an exemplary method for producing anickel-manganese composite hydroxide of the present embodiment. In thefollowing, in describing FIG. 2 , FIG. 1 , which is a schematic diagramof an exemplary composite hydroxide 1, is referred to as appropriate.

As illustrated in FIG. 2 , the production method of the compositehydroxide 1 of the present embodiment includes the crystallizationprocess in which a salt containing at least nickel and a salt containingat least manganese are neutralized to effect the co-precipitation in thereaction aqueous solution in a crystallization reaction vessel. In thepresent embodiment, during the crystallization process, it is importantto adjust the dissolved nickel concentration and the dissolved oxygenconcentration in the reaction aqueous solution to fall within thespecific ranges. By adjusting these factors (parameters), each of theparticle diameter d of the secondary particle 3 to be obtained and thedegree of sparsity/density inside the secondary particle 3 can becontrolled. In addition, by adjusting the stirring power loaded to thereaction aqueous solution, crystallinity of the secondary particle 3 andthe degree of sparsity/density inside the particle 3 can be controlledfurther precisely.

The inventors of the present invention intensively studied productionconditions of the composite hydroxide 1 and have found out that themorphology of the primary particles 2 and the secondary particle 3 canbe accurately controlled by adjusting the dissolved oxygen concentrationand the dissolved nickel concentration in the reaction aqueous solution.That is to say, the method of production of the present embodiment canproduce a composite hydroxide 1 used suitably also as the precursor ofthe positive electrode active material by adjusting the dissolved nickelconcentration to fall within the specific ranges in accordance with thedissolved oxygen concentration. The “morphology” refers tocharacteristics related to the form and structure of the primaryparticles 2 and/or the secondary particle 3 including the shape, thedegree of sparsity/density, the average particle diameter, the particlesize distribution, the crystal structure, and the tap density of theparticles.

In the production method of the composite hydroxide 1 of the presentembodiment, when the dissolved oxygen concentration in the reactionaqueous solution is adjusted within a comparatively low range, andfurther the dissolved nickel concentration is adjusted within acomparatively high range, the crystallization rate of the primaryparticle 2 decreases; and thus, by increasing the thickness of theprimary particle 2 so as to fill the void 4 among the primary particles2, the secondary particle 3 having a dense structure can be formed. Inaddition, when the dissolved nickel concentration is adjusted within ahigh range, coarsening of the particle diameter of the secondaryparticle 3 can be suppressed.

Furthermore, the method for producing the composite hydroxide 1 of thepresent embodiment controls a flocculated state of the primary particles2 by stirring power in the reaction aqueous solution, whereby theparticle diameter of the secondary particle 2 can be controlled moreaccurately in a wide range. That is to say, when the dissolved oxygenconcentration is adjusted to fall within a low range, the stirring poweris controlled to fall within a high range, whereby coarse growth of thesecondary particle 3 due to the flocculation of the primary particles 2can be inhibited. In addition, the secondary particle 3 is inhibitedfrom increasing in diameter, whereby the precipitation of the compositehydroxide within the secondary particle 3 is facilitated, and thesecondary particle 3 can be made denser. The following describesconditions on the method the method for producing the compositehydroxide 1 of the present embodiment.

(Dissolved Oxygen Concentration)

The dissolved oxygen concentration in the reaction aqueous solution isadjusted in the range of at least 0.2 mg/L and up to 4.6 mg/L. When thedissolved oxygen concentration is controlled to fall within the rangedescribed above, by controlling the degree of sparsity/density of thesecondary particle 3 within the range described above, the compositehydroxide that is suitable as the precursor of the positive electrodeactive material can be obtained. In addition, in the crystallizationprocess, it is preferable that the dissolved oxygen concentration becontrolled to fall within a certain range. The fluctuation width of thedissolved oxygen concentration is preferably, for example, within ±0.2mg/L and more preferably within ±0.1 mg/L.

When the dissolved oxygen concentration is within the range describedabove, the composite hydroxide 1 having a dense structure and a highfilling density (tap density) as can be seen in, for example, FIG. 5Aand FIG. 5B, can be obtained. Because the positive electrode activematerial that is produced using this composite hydroxide 1 has a highfilling density, a high battery capacity can be obtained. When thedissolved oxygen concentration is less than 0.2 mg/L, oxidation of thetransition metals, especially oxidation of manganese, hardly takesplace; as a result, inside the secondary particle 3 becomes extremelydense. In addition, the surface thereof may have a peculiar form. In thepositive electrode active material obtained by using the compositehydroxide like this, the reaction resistance increases and the outputcharacteristic deteriorates. On the other hand, when the dissolvedoxygen concentration is more than 4.6 mg/L, the secondary particlethereby produced has a further sparse structure.

Here, the dissolved oxygen concentration may be measured by a methodsuch as a Winkler method (chemical analysis method), a diaphragmpermeation method (electrochemical measurement method), or afluorescence measurement method. The same measurement value of thedissolved oxygen concentration can be obtained with these measurementmethods; and thus, any method mentioned above may be used. The dissolvedoxygen concentration in the reaction aqueous solution can be adjusted byintroducing into a reaction vessel a gas such as, for example, an inertgas (for example, N₂ gas and Ar gas), an air, or oxygen, withcontrolling the flow rates or the composition of these gases. Here,these gases may be flowed into the space of the reaction vessel or blowninto the reaction aqueous solution. By appropriately stirring thereaction aqueous solution by using the stirring equipment such as astirring blade with the power within the range to be described later,the dissolved oxygen concentration in the whole reaction aqueoussolution can be made further uniform.

(Dissolved Nickel Concentration)

The dissolved nickel concentration in the reaction aqueous solution isadjusted, for example, in the range of at least 700 mg/L and up to 1,500mg/L, and preferably in the range of at least 700 mg/L and up to 1,200mg/L, based on the temperature of the reaction aqueous solution. Whenthe dissolved nickel concentration is appropriately adjusted within therange described above, the average particle diameter and the degree ofsparsity/density can be controlled to fall within the desired respectiveranges, so that the nickel-manganese composite hydroxide having a lowdegree of sparsity/density and a high sphericity as the precursor of thepositive electrode active material can be easily obtained. In addition,in the crystallization process, it is preferable to control thedissolved nickel concentration so as to be within a certain range. It ispreferable that the fluctuation range of the dissolved nickelconcentration be, for example, within ±20 mg/L. The dissolved nickelconcentration may be measured, for example, by chemically analyzing theNi amount in a liquid component of the reaction aqueous solution with anICP emission spectrometry.

When the dissolved nickel concentration in the reaction aqueous solutionis less than 700 mg/L, the growth rate of the primary particle 2 is sofast that there is a tendency that the nucleus generation is dominantover the particle growth, so that the degree of sparsity/density of thesecondary particle is prone to become higher than the range describedabove. On the other hand, when the dissolved nickel concentration ismore than 1,500 mg/L, the generation rate of the composite hydroxide 1(secondary particle 3) becomes extremely slow so that nickel remains inthe filtrate thereby occasionally causing significant deviation of thecomposition of the obtained composite hydroxide 1 from the target valuesthereof. In addition, under the condition that the dissolved nickelconcentration is too high, impurities included in the compositehydroxide 1 significantly increase thereby occasionally causingdeterioration of the battery characteristics when the positive electrodeactive material obtained from the composite hydroxide is used in thebattery.

(Stirring Power)

The stirring power loaded to the reaction aqueous solution is adjustedpreferably in the range of at least 3 kW/m³ and up to 15 kW/m³, morepreferably in the range of at least 3 kW/m³ and up to 14 kW/m³, and farpreferably in the range of at least 4.5 kW/m³ and up to 12 kW/m³. Whenthe stirring power is made within the range described above, excessiverefinement or coarsening of the secondary particle can be suppressed, sothat the particle diameter of the composite hydroxide 1 can be madefurther suitable as the positive electrode active material. In addition,by suppressing coarsening of the secondary particle, the secondaryparticle can be made further dense. In the crystallization process, itis preferable to control the stirring power so as to be within a certainrange. The fluctuation width of the stirring power may be made, forexample, within ±0.2 kW/m³. Alternatively, the stirring power may beadjusted, for example, in the range of up to 7 kW/m³, or in the range ofup to 6.5 kW/m³. The stirring power is adjusted to fall within the rangedescribed above by adjusting the size, the rotation number, and the likeof the stirring equipment such as a stirring blade disposed in thereaction vessel.

When the stirring power is less than 3 kW/m³, the primary particles 2are prone to be flocculated, so that the coarsened secondary particle 3may be formed. With this, the fillability of the positive electrodeactive material may decrease. On the other hand, when the stirring poweris more than 15 kW/m³, flocculation of the primary particles is prone tobe excessively suppressed, so that the secondary particle 3 becomes toosmall, thereby occasionally resulting in decrease of the fillability ofthe positive electrode active material.

(Reaction Temperature)

The temperature of the reaction aqueous solution in the crystallizationreaction tank is preferably within a range of at least 35° C. and up to60° C. and more preferably within a range of at least 38° C. and up to50° C. When the temperature of the reaction aqueous solution is greaterthan 60° C., the degree of priority of nucleation increases overparticle growth in the reaction aqueous solution, and the shape of theprimary particles 2 forming the composite hydroxide 1 is likely to beextremely fine. Use of such a composite hydroxide 1 causes a problem inthat the fillability of the positive electrode active material to beobtained degrades. In contrast, when the temperature of the reactionaqueous solution is less than 35° C., particle growth tends to bepreferential over nucleation in the reaction aqueous solution, and theshapes of the primary particles 2 and the secondary particle 3 formingthe composite hydroxide 1 are likely to increase in size. Use of thecomposite hydroxide having such a coarse secondary particle 3 as theprecursor of the positive electrode active material causes a problem inthat the positive electrode active material containing so extremelylarge coarse particles that irregularities occur during electrodeproduction is formed. Furthermore, the reaction aqueous solution beingless than 35° C. causes a problem in that a remaining amount of metalions in the reaction aqueous solution is large, and reaction efficiencyis extremely bad and is likely to cause a problem in that a compositehydroxide containing a large amount of impurity elements is generated.

(pH Value)

The pH value of the reaction aqueous solution is preferably within arange of at least 10.0 and up to 13.0 with a liquid temperature of 25°C. as a basis. When the pH value is within the above range, themorphology of the secondary particle is appropriately controlled whilecontrolling the degree of sparsity/density by appropriately controllingthe size and shape of the primary particles 2, and thus the compositehydroxide 1 more suitable as the precursor of the positive electrodeactive material can be obtained. When the pH value is less than 10.0,the generation rate of the composite hydroxide 1 is extremely lowered,nickel remains in the filtrate, and the composition of the compositehydroxide 1 to be obtained may be substantially deviated from the targetvalue. In contrast, when the pH value is greater than 13.0, the growthrate of the particles is high, nucleation is likely to occur, andparticles with a small diameter and less sphericity are likely to beformed.

(Others)

The method of production of the present embodiment includes thecrystallization process that generates nickel-manganese compositehydroxide particles by neutralizing a salt containing at least nickeland a salt containing at least manganese in the reaction aqueoussolution. As a specific embodiment of the crystallization process, aneutralizer (e.g., an alkali solution) is added to a mixed aqueoussolution containing at least nickel (Ni) and manganese (Mn) in thereaction tank while stirring at a constant speed to performneutralization, whereby pH is controlled, and the composite hydroxide 1particle can be generated through coprecipitation, for example. Themethod of production of the present embodiment can employ any method ofa batch type method of crystallization and a continuous method ofcrystallization. The continuous method of crystallization is a method ofcrystallization that supplies a neutralizer while continuously supplingthe mixed aqueous solution to control pH and collects compositehydroxide particles generated by overflow. The continuous method ofcrystallization obtains particles having wider particle sizedistribution and easily obtains particles having higher fillability thanthe batch method. In addition, the continuous method of crystallizationis suitable for mass production and is an advantageous method ofproduction also industrially. When the composite hydroxide 1 of thepresent embodiment described above is produced by the continuous methodof crystallization, for example, the fillability (tap density) of thecomposite hydroxide 1 particle to be obtained can be improved, and thecomposite hydroxide 1 having higher fillability and a degree ofsparsity/density can be produced simply and in a large amount.

For the mixed aqueous solution, an aqueous solution containing at leastnickel and manganese, that is to say, an aqueous solution dissolving atleast a nickel salt and a manganese salt can be used. Furthermore, themixed aqueous solution may contain M, and an aqueous solution dissolvinga nickel salt, a manganese salt, and a salt containing M may be used.For a nickel salt, a manganese salt, a salt containing and M, at leastone selected from the group consisting of sulfates, nitrates, andchlorides can be used, for example. Among them, sulfates are preferablyused in view of costs and liquid-waste treatment.

The concentration of the mixed aqueous solution is preferably at least1.0 mol/L and up to 2.4 mol/L and more preferably at least 1.2 mol/L andup to 2.2 mol/L in terms of the total of the dissolved metal salts. Whenthe concentration of the mixed aqueous solution is less than 1.0 mol/Lin terms of the total of the dissolved metal salts, the concentration isextremely low, and the primary particles 2 forming the compositehydroxide 1 (the secondary particle 3) may fail to sufficiently grow. Incontrast, when the concentration of the mixed aqueous solution isgreater than 2.4 mol/L, it is greater than a saturated concentration atroom temperature, and crystals are reprecipitated, which may cause therisk of clogging of piping or the like. In addition, in this case, thenucleation amount of the primary particles 2 increases, and theproportion of fine particles within the composite hydroxide particles tobe obtained may increase. The composition of the metal elementscontained in the mixed aqueous solution matches the concentration of themetal elements contained in the composite hydroxide 1 to be obtained.Consequently, the composition of the metal elements of the mixed aqueoussolution can be prepared so as to match the composition of the metalelements of the target composite hydroxide 1.

Together with the neutralizer, a complexing agent may be added to themixed aqueous solution. The complexing agent is not limited to aparticular agent and may be any one that can forma complex throughbonding to metal elements such as nickel ions and manganese ions in anaqueous solution; examples of the complexing agent include an ammoniumion supplier. For the ammonium ion supplier, which is not limited to aparticular substance, at least one selected from the group consisting ofammonia water, an aqueous ammonium sulfate solution, and an aqueousammonium chloride solution can be used, for example. Among them, ammoniawater is preferably used in view of handleability. When the ammonium ionsupplier is used, the concentration of ammonium ions is preferablywithin a range of at least 5 g/L and up to 25 g/L.

For the neutralizer, an alkali solution can be used; general aqueousalkali metal hydroxide solutions such as sodium hydroxide and potassiumhydroxide can be used, for example. Among them, an aqueous sodiumhydroxide solution is preferably used in view of costs andhandleability. Although an alkali metal hydroxide can be directly addedto the reaction aqueous solution, it is preferably added as an aqueoussolution in view of easiness of pH control. In this case, theconcentration of the aqueous alkali metal hydroxide solution ispreferably at least 12% by mass and up to 30% by mass and morepreferably at least 20% by mass and up to 30% by mass. When theconcentration of the aqueous alkali metal hydroxide solution is lessthan 12% by mass, a supply amount to the reaction tank increases, andparticles may fail to sufficiently grow. In contrast, when theconcentration of the aqueous alkali metal hydroxide solution is greaterthan 30% by mass, the pH value increases locally at an addition positionof the alkali metal hydroxide, and fine particles may be generated.

The method of production of the present embodiment preferably includes awashing process after the crystallization process. The washing processis a process that washes away impurities contained in the compositehydroxide 1 obtained in the crystallization process. For a washingsolution, pure water is preferably used. The amount of the washingsolution is preferably at least 1 L relative to 300 g of the compositehydroxide 1. When the amount of the washing solution is less than 1 Lrelative to 300 g of the composite hydroxide 1, washing is insufficient,and the impurities may remain in the composite hydroxide 1. As to amethod of washing, the washing solution such as pure water may be passedthrough a filter such as a filter press, for example. When SO₄ remainingin the composite hydroxide 1 is desired to be further washed away,sodium hydroxide, sodium carbonate, or the like is preferably used asthe washing solution.

(3) Positive Electrode Active Material for Nonaqueous ElectrolyteSecondary Battery

FIG. 3(A) is a schematic drawing that illustrates one example of thelithium-nickel-manganese composite oxide 11 (hereinafter, this is alsoreferred to as “composite oxide 11”) that constitutes the positiveelectrode active material 10 for a nonaqueous electrolyte secondarybattery of the present embodiment (hereinafter, this positive electrodeactive material fora nonaqueous electrolyte secondary battery is alsoreferred to as “positive electrode active material 10”). FIG. 3(B) is anexplanatory drawing of disposition of the primary particles 12 in thepositive electrode active material 10 (secondary particle 13). Thecomposite oxide 11 is represented by General Formula (2):Li_(1+t)Ni_(x)Mn_(y)M_(z)O_(2+β) (in Formula (2), M is at least oneadditional element selected from Co, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Fe,and W; and t, x, y, z, and β satisfy −0.05≤t≤0.5, 0.1≤x≤0.9, 0.05≤y≤0.8,0≤z≤0.8, x+y+z=1.0, and 0≤β≤0.5) and contains the secondary particle 13formed of a plurality of flocculated primary particles 12. Here, inFormula (2), β is a coefficient that varies in accordance with thevalencies of the metal elements other than lithium and with the ratio ofthe lithium atom to the metal elements other than lithium included inthe composite oxide 11.

As described below, the composite oxide 11 is formed by mixing thecomposite hydroxide 1 described above and a lithium compound togetherand firing the mixture. Consequently, the composition of the compositeoxide 11 is substantially the same as that of the composite hydroxide 1except lithium. In Formula (2), y and z preferably satisfy 0.1≤y≤0.8and, when M is Co, preferably satisfy 0.1≤z≤0.4 for the same reason as yand z of Formula (1), for example.

The positive electrode active material 10 of the present embodimentusing the composite hydroxide 1 as the precursor thereof can give thesecondary battery having a very high energy density and a sufficientoutput characteristic. Here, although the composite oxide 11 mainlycontains the secondary particle 13 formed of a plurality of flocculatedprimary particles 12, this may also include small amount of the primaryparticle 12 (independent), as in the case of the composite hydroxide 1.In addition, the positive electrode active material 10 may includelithium-metal composite oxides other than the composite oxide 11 so faras the effects of the present invention are not impaired. Hereinafter,each characteristic of the positive electrode active material 10 will beexplained.

In the positive electrode active material 10, the degree ofsparsity/density is at least 0.5% and up to 12%, and preferably at least1.0% and up to 10%. When the degree of sparsity/density is within therange described above, the electrolyte solution can penetrate intoinside the secondary particle 13 sufficiently well, so that a highbattery capacity and a good output characteristic can be obtained; andin addition, the secondary particle 13 can be made to a dense state sothat a high fillability can be obtained. Therefore, when the compositeoxide 11 like this is used as the positive electrode active material inthe secondary battery, the secondary battery having a high volume energydensity and a sufficient output characteristic can be obtained. When thedegree of sparsity/density is less than 0.5%, penetration of theelectrolyte solution into inside the secondary particle is insufficientso that a high battery capacity cannot be obtained. Accordingly, even ifthe fillability into the battery container is high, the battery capacityof each particle decreases; and thus, the energy density as the wholeactive material decreases.

Here, “degree of sparsity/density” is the value obtained from the resultof the image analysis of the cross section of the particle of thecomposite oxide 11 by using a scanning electron microscope (SEM), andthis is the value represented by [(area of the void 14 inside thesecondary particle 13/area of the cross section of the secondaryparticle 13)×100](%). For example, in the cross section of the particleof the composite oxide 11 depicted in FIG. 3 , the degree ofsparsity/density is the value represented by [(area of the void 14)/(sumof the area of the cross section of the primary particle 12 and the areaof void 14)×100]. Here, similarly to the particle of the compositehydroxide 1, the degree of sparsity/density is an average degree ofsparsity/density obtained by measuring each cross section of 20particles of the secondary particles 13.

In the positive electrode active material 10, the DBP absorption amount(hereinafter, this is also referred to as “oil absorption amount”)measured in accordance with JIS K6217-4:2008 is at least 12 cm^(3/100) gand up to 20 cm^(3/100) g. When the oil absorption amount is within therange described above, the secondary battery using the positiveelectrode active material 10 in the positive electrode can retain asufficient amount of the electrolyte solution in the positive electrodeso that migration of a lithium ion intervened by the electrolytesolution is not limited, and thus, a sufficient battery capacity can beobtained. When the oil absorption amount is less than 12 cm^(3/100) g,the electrolyte solution retained in the positive electrode isinsufficient, so that the battery capacity and the output characteristicdeteriorate.

In the positive electrode active material 10, the ratio I(003)/I(104) ofthe diffraction peak intensity I(003) of the 003 plane to the peakintensity I(104) of the 104 plane (hereinafter, this ratio is alsoreferred to as “peak intensity ratio”), obtained by X-ray diffractionmeasurement, is preferably at least 1.7, and more preferably at least1.7 and up to 2.5. When the peak intensity ratio is at least 1.7,crystallinity of the positive electrode active material 10 is high sothat the battery capacity and the output characteristic can be enhanced.In addition, when the peak intensity ratio is within the range describedabove, the primary particle grows in a specific crystal plane so thatorientation of the primary particle 12 in the secondary particle 13 isenhanced thereby leading to a structure that at least some of theprimary particles 12 are radially disposed from the central part C ofthe secondary particle 13 to the outer circumference thereof (radialstructure). Because of the radial structure, penetration of theelectrolyte solution into inside the positive electrode active material10 is facilitated, and in addition, the stress load due to expansion andshrinkage of the positive electrode active material 10 caused uponcharging and discharging can be relaxed in the particle boundary of theprimary particles 12; and thus, the cycle characteristic can beenhanced.

In the primary particle 12 in the secondary particle 13 (positiveelectrode active material 10), for example, in the region R2 that is 50%of the radius from the outer circumference of the secondary particle 13to the central part C of the particle (see FIG. 3(B)), it is preferablethat at least 50% of the primary particles 12 in number relative to thetotal number of the primary particles 12 that are present in the 50%region be radially disposed from the central part C of the secondaryparticle 13 to the outer circumference thereof. With this, the positiveelectrode active material 10 can have the particle structure having afurther enhanced radial orientation (radial structure); and thus, whenused in the positive electrode of the secondary battery, the batterycharacteristics can be enhanced furthermore. In order to enhance thebattery characteristics furthermore, it is more preferable that at least70% of the primary particles 12 be radially disposed in the 50% radiusregion R2. When the dissolved oxygen concentration is within the rangedescribed before, by adjusting, for example, the stirring power togetherwith the dissolved nickel concentration, the primary particle 12 can bedisposed radially with a higher ratio thereof. For example, when the Niconcentration is adjusted in the range of at least 700 mg/L and up to1,500 mg/L, and the stirring power in the range of at least 4.0 kW/m³and up to 12.0 kW/m³, the radial structure can be realized furthereminently.

Here, to be disposed radially means, for example, as can be seen in FIG.3(B), the state in which the direction of the long diameter L of theprimary particle 12 in the cross section of the composite hydroxide 11is orientated in the radial direction R1 from the central part C of thesecondary particle 13 to the outer circumference thereof. Here, to beorientated in the radial direction R1 means that in the cross section ofthe composite hydroxide 11, the angle difference θ between the directionof the long diameter L of the primary particle 12 and the radialdirection R1 is within up to 45°, and preferably within up to 30°. Ascan be seen, for example, in FIG. 3(B), the angle difference θ betweenthe direction of the long diameter L of the primary particle 12 and theradial direction R1 may be obtained from the angle between, among theradial directions from the central part C of the secondary particle 13to the outer circumference thereof, the radial direction R1 that passesthrough the center of the long diameter and the direction of the longdiameter L, in which the direction of the long diameter L is thedirection from one end near the central part of the secondary particle13 to the other end in the long diameter of the primary particle 12.

The positive electrode active material 10 has a tap density ofpreferably within a range of at least 2.0 g/cm³ and up to 2.7 g/cm³ andpreferably within a range of at least 2.2 g/cm³ and up to 2.5 g/cm³.When the tap density is within the above range, the positive electrodeactive material achieves both excellent battery capacity andfillability, and battery energy density can be further improved.

The positive electrode active material 10 has a volume-average particlediameter MV preferably of at least 5 μm and up to 20 μm and morepreferably of at least 6 μm and up to 15 μm. When the volume-averageparticle diameter MV is within the above range, the specific surfacearea is inhibited from reducing while fillability is maintained at ahigh level, and a battery using this positive electrode active materialcan achieve both high filling density and excellent outputcharacteristics.

Furthermore, the positive electrode active material 10 preferably has[(D90−D10)/an average particle diameter] indicating a particle diametervariation index of at least 0.70. When the variation index of thepositive electrode active material 10 is within the above range, fineparticles and coarse particles appropriately mix, and particlefillability can be further improved while the cycle characteristics andoutput characteristics of the positive electrode active material 10 tobe obtained are inhibited from degrading. In view of inhibitingexcessive mixing of fine particles or coarse particles into the positiveelectrode active material 10, the variation index of the positiveelectrode active material 10 is preferably up to 1.2 and more preferablyup to 1.0.

FIG. 7(A) and FIG. 7(B) are the drawings to explain the evaluationmethod of the crystal orientation of the positive electrode activematerial 10 by using the electron backscatter diffraction method (EBSD).In EBSD, by using a scanning electron microscope (SEM), an electron beamis irradiated to a sample, and the Kikuchi pattern thereby formed in thesample's plane to be measured due to a diffraction phenomenon of theelectron beam is analyzed so that the crystal direction and the like ofa minute portion thereof can be measured. By analyzing the crystaldirection measured by EBSD, the crystal orientation in a specificdirection can be evaluated.

In this specification, an arbitrary radial direction from the center C2of a cross section of the secondary particle 13 (see FIG. 7(A)), whichconstitutes the positive electrode active material 10, toward an outercircumference thereof is regarded as an x-axis direction and a directionperpendicular to the x-axis direction is regarded as a y-axis direction,and whereby an evaluation of the crystal orientation using EBSD iscarried out. Hereinafter, each direction will be explained by referringto FIG. 7(A).

For example, as illustrated in FIG. 7(A), when the observation crosssection is regarded as the paper surface, the x-axis direction in thecross section of the secondary particle 13 is the direction toward ahorizontal direction from the center C2 in the observation crosssection. Also, when the observation cross section is regarded as thepaper surface, the y-axis direction in the cross section of thesecondary particle 13 is the direction toward a perpendicular directionfrom the center C2 in the observation cross section. When theobservation cross section is regarded as the paper surface, the z-axisdirection is the vertical direction and the front direction from thecenter C2 in the observation cross section.

In the positive electrode active material 10 according to the presentembodiment, the orientation rate of the crystal ab plane measured byEBSD in each of the x-axis direction and the y-axis direction ispreferably at least 55%, more preferably at least 58%, and farpreferably at least 60%. When the orientation rate of the crystal abplane is within the range described above, the battery capacity can beenhanced furthermore.

The lithium-nickel-manganese composite oxide (positive electrode activematerial 10) has a hexagonal crystal structure, and it also has alayered structure in which a transition metal ion layer formed ofnickel, manganese, and the like and a lithium ion layer are alternatelystacked in a c-axis direction. Here, when the secondary battery ischarged and discharged, the lithium ion in the crystal that constitutesthe positive electrode active material 10 migrates to the [100]-axisdirection or to the [110]-axis direction (ab plane), and thereby thelithium ion is inserted and de-inserted. Accordingly, although thedetail is not yet clear, it is presumed that when the orientation ratesof the crystal ab plane in the x-axis direction and the y-axis directioneach are within the range described above, insertion and de-insertion oflithium ions can be facilitated furthermore in the positive electrodeactive material 10 thereby leading to a further increase in the batterycapacity.

On the other hand, for example, in the positive electrode activematerial having the structure in which the primary particles arerandomly flocculated, the orientation rate of the crystal ab plane in atleast one of the x-axis direction and the y-axis direction is less than55%, while the orientation rate in the c-axis direction increases. Inthis case, the battery capacity may be insufficient in the positiveelectrode active material 10 in the secondary battery (positiveelectrode).

Here, EBSD-based evaluation is carried out as follows. Namely, in thecross-sectional observation of the secondary particle 13, at least 3 ofthe secondary particle 13 that is at least 80% of the volume-averageparticle diameter (MV) are selected; and the orientation rates of the abplane in the x-axis direction and the y-axis direction of each particleare measured followed by averaging these measured values. With regard tothe specific evaluation method by EBSD, the method described in Exampleto be described later may be used.

(4) Method for Producing the Positive Electrode Active Material for aNonaqueous Electrolyte Secondary Battery

The production method of the present embodiment to produce the positiveelectrode active material for a nonaqueous electrolyte secondary battery(hereinafter, this material is also referred to as “positive electrodeactive material”) is the method in which the material is alithium-nickel-manganese composite oxide that is represented by GeneralFormula (2): Li_(1+t)Ni_(x)Mn_(y)M_(z)O_(2+β) (in Formula (2), M is atleast one additional element selected from Co, Ti, V, Cr, Zr, Nb, Mo,Hf, Ta, Fe, and W; and t, x, y, z, and β satisfy −0.05≤t≤0.5, 0.1≤x≤0.9,0.05≤y≤0.8, 0≤z≤0.8, x+y+z=1.0, and 0≤β≤0.5) and contains the secondaryparticle formed of a plurality of flocculated primary particles.

FIG. 4 is a diagram of an example of the method for producing a positiveelectrode active material 10 of the present embodiment. As illustratedin FIG. 4 , the method for producing the positive electrode activematerial 10 includes a process of obtaining a mixture by mixing thecomposite hydroxide 1 and a lithium compound together and a firingprocess of obtaining a composite oxide 11 by firing the mixture. Themorphology of the composite oxide 11 is strongly influenced by themorphology of the composite hydroxide 1 as a precursor. For this reason,the powder characteristics of the composite hydroxide 1 are adjusted tofall within the specific ranges as described above, whereby the powdercharacteristics of the composite oxide 11 can be controlled to fallwithin the specific ranges. The following describes the method forproducing the positive electrode active material 10.

(Mixing Process)

First, the composite hydroxide 1 is mixed with a lithium compound toform a lithium mixture. It is preferable that the composite hydroxide 1be obtained by the production method described above. The lithiumcompound is not particularly limited, and heretofore known lithiumcompounds may be used. For example, in view of easy availability,lithium hydroxide, lithium nitrate, lithium carbonate, or a mixture ofthese may be preferably used. Among them, in view of easy handling andstable quality, lithium hydroxide or lithium carbonate are morepreferable as the lithium compound. Here, prior to the mixing process,the composite hydroxide 1 may be oxidized thereby converting it to aform of a nickel-manganese composite oxide, and then, this may be mixedwith the lithium compound.

The composite hydroxide 1 and the lithium compound are mixed togethersuch that the ratio (Li/Me) between the atom number of metals other thanlithium in the lithium mixture, that is to say, the sum (Me) of the atomnumbers of nickel, cobalt, and an additional element and the atom number(Li) of lithium is at least 0.95 and up to 1.50 and preferably at least0.95 and up to 1.20. That is to say, Li/Me does not change before andafter firing, and the Li/Me ratio mixed in this mixing process is aLi/Me ratio in the positive electrode active material, and Li/Me in thelithium mixture is mixed so as to be the same as Li/Me in the positiveelectrode active material to be obtained.

For mixing, general mixers can be used; examples thereof include shakermixers, Loedige mixers, Julia mixers, and V blenders. Mixing may beperformed sufficiently to the extent that the skeleton of the compositehydroxide 1 is not destroyed.

(Firing Process)

Next, the lithium compound is fired to obtain a composite oxide 11. Thefiring is performed in an oxidative atmosphere at at least 700° C. andup to 1,100° C. When the firing temperature is less than 700° C., firingis not sufficiently performed, and the tap density may reduce. Inaddition, when the firing temperature is less than 700° C., diffusion oflithium does not sufficiently proceed, surplus lithium remains, and acrystal structure may fail to be well-regulated, or the uniformity ofthe composition of nickel, manganese, and the like within the particlecannot be sufficiently obtained, and sufficient characteristics cannotnecessarily be obtained when used for a battery. In contrast, when thefiring temperature is greater than 1,100° C., a sparse part on aparticle surface is made dense. In addition, sintering may fiercelyoccur among particles of the composite oxide 11, abnormal particlegrowth may occur, and consequently, particles after firing may increasein size and may fail to hold their substantially spherical secondaryparticle form. Such a positive electrode active material reduces in thespecific surface area and thus causes a problem in that the resistanceof a positive electrode increases to reduce battery capacity when usedfor a battery. The time for firing, which is not limited to a particulartime, is about at least 1 hour and up to 24 hours.

In view of uniformly conducting a reaction of the composite hydroxide 1or the composite oxide 11 obtained by oxidizing it and the lithiumcompound, the temperature is preferably raised up to the firingtemperature with a temperature raising rate within a range of at least1° C./min and up to 10° C./min, for example. Furthermore, before firing,the lithium compound may be held at a temperature near the melting pointof the lithium compound for about 1 hour to 10 hours. With this, thereaction can be conducted more uniformly.

In the method for producing the positive electrode active material 10 ofthe present embodiment, the composite hydroxide 1 used may containsingle primary particles 2 such as a primary particle 2 that has notbeen flocculated as the secondary particle 3 and a primary particle 2that has fallen from the secondary particle 3 after being flocculatedother than the composite hydroxide 1 including the secondary particle 3formed of the flocculated primary particles 2. The composite hydroxide 1used may contain a composite hydroxide produced by a method other themethod described above or a composite oxide obtained by oxidizing thecomposite hydroxide to the extent that the effects of the presentinvention are not impaired.

(5) Nonaqueous Electrolyte Secondary Battery

The following describes an example of a nonaqueous electrolyte secondarybattery (hereinafter, also referred to as a “secondary battery”) of thepresent embodiment for each component. The secondary battery of thepresent embodiment includes a positive electrode, a negative electrode,and a nonaqueous electrolyte solution and includes components similar tothose of general lithium ion secondary batteries. The embodimentdescribed below is only by way of example, and the nonaqueouselectrolyte secondary battery can be performed with forms to whichvarious modifications and improvements have been made based on theknowledge of those skilled in the art including the followingembodiment. The secondary battery is not limited to particular uses.

(Positive Electrode)

Using the positive electrode active material 10, the positive electrodeof the nonaqueous electrolyte secondary battery is produced. Thefollowing describes an example of a method for manufacturing thepositive electrode. First, the positive electrode active material 10(powdery), an electric conductor, and a binding agent (binder) are mixedtogether, activated carbon as needed and a solvent for viscosityadjustment or the like are further added thereto, and this mixture iskneaded to produce a positive electrode mixture paste.

The mixture ratio of the materials in the positive electrode mixture isa factor for determining the performance of a lithium secondary batteryand can thus be adjusted in accordance with uses. The mixture ratio ofthe materials can be similar to that of a positive electrode of knownlithium secondary batteries; when the total mass of the solid content ofthe positive electrode mixture excluding the solvent is 100% by mass, 60to 95% by mass of the positive electrode active material, 1 to 20% bymass of the electric conductor, and 1 to 20% by mass of the bindingagent can be contained, for example.

The obtained positive electrode mixture paste is applied to the surfaceof a collector made of aluminum foil and is dried to scatter the solventto produce a sheet-shaped positive electrode, for example. As needed,pressurizing may be performed using a roll press or the like in order toincrease electrode density. The thus obtained sheet-shaped positiveelectrode is cut or the like into appropriate size in accordance with atarget battery to be served for production of the battery. However, themethod for producing the positive electrode is not limited to theexemplified one and may be another method.

Examples of the electric conductor include graphite (natural graphite,artificial graphite, expanded graphite, and the like) and carbon blackmaterials such as acetylene black and Ketjen black.

Examples of the binding agent (binder), which plays a role of bindingactive material particles, include polyvinylidene fluoride (PVDF),polytetrafluoroethylene (PTFE), fluoro rubber, ethylene-propylenerubber, styrene butadiene, cellulosic resins, and polyacrylic acid.

As needed, a solvent for dispersing the positive electrode activematerial, the electric conductor, and the activated carbon anddissolving the binding agent is added to the positive electrode mixture.Specific examples of the solvent include organic solvents such asN-methyl-2-pyrrolidone. The activated carbon can be added to thepositive electrode mixture in order to increase electric double layercapacity.

(Negative Electrode)

Examples of the negative electrode include metal lithium, lithiumalloys. The negative electrode may be formed by applying a negativeelectrode mixture obtained by mixing a binding agent with a negativeelectrode active material that can occlude and desorb lithium ions andadding an appropriate solvent to be paste form to the surface of a metalfoil collector such as copper, drying, and compressing it in order toincrease electrode density as needed.

Examples of the negative electrode active material include naturalgraphite, artificial graphite, organic compound fired bodies such asphenol resin, and powder of carbon substances such as coke. In thiscase, examples of a negative electrode binding agent includefluorine-containing resins such as PVDF similarly to the positiveelectrode. Examples of a solvent in which the active material and thebinding agent are dispersed include organic solvents such asN-methyl-2-pyrrolidone.

(Separator)

A separator is interposed between the positive electrode and thenegative electrode. The separator separates the positive electrode andthe negative electrode from each other and holds an electrolyte;examples thereof include thin films formed of polyethylene,polypropylene, or the like, the films having many minute holes.

(Nonaqueous Electrolyte Solution)

A nonaqueous electrolyte solution is a solution obtained by dissolving alithium salt as a supporting salt in an organic solvent. Examples of theorganic solvent include cyclic carbonates such as ethylene carbonate,propylene carbonate, butylene carbonate, and trifluoro propylenecarbonate; chain carbonates such as diethyl carbonate, dimethylcarbonate, ethylmethyl carbonate, and dipropyl carbonate; ethercompounds such as tetrahydrofuran, 2-methyltetrahydrofuran, anddimethoxy ethane; sulfur compounds such as ethylmethyl sulfone andbutane sulfone; and phosphorous compounds such as triethyl phosphate andtrioctyl phosphate; for the solvent, one or two or more in combinationselected from the above can be used.

Examples of the supporting salt include LiPF₆, LiBF₄, LiClO₄, LiAsF₆,LiN(CF₃SO₂)₂ and their composite salts. Furthermore, the nonaqueouselectrolyte solution may contain radical scavengers, surfactants, fireretardants, and the like.

(Shape and Configuration of Battery)

The nonaqueous electrolyte secondary battery of the present inventionincluding the positive electrode, the negative electrode, the separator,and the nonaqueous electrolyte solution described above can be formedinto various shapes such as cylindrical and stacked shapes. For anyshape employed, the positive electrode and the negative electrode arestacked via the separator to form an electrode body, the obtainedelectrode body is impregnated with the nonaqueous electrolyte solution,a positive electrode collector and a positive electrode terminalcommunicating with the outside and a negative electrode collector and anegative electrode terminal communicating with the outside are eachconnected using a collector lead, and the electrode body is hermeticallysealed in a battery case to complete the nonaqueous electrolytesecondary battery.

EXAMPLES

The following describes specific examples of the present invention. Thepresent invention, however, is not limited to these examples.

Example 1

[Preparation of Composite Hydroxide]

A prescribed amount of purified water was taken into a reaction vessel(60 L), and the stirring power was adjusted at 6.0 kW/m³. Next, withstirring, the temperature inside the reaction vessel (liquidtemperature) was set at 45° C. At this time, a nitrogen gas (N₂) was fedin such a way that the dissolved oxygen concentration in the solution inthe reaction vessel became 2.8 mg/L by adjusting the N₂ flow rate. Intothis reaction vessel, a 2.0 mol/L mixed aqueous solution dissolvingnickel sulfate, cobalt sulfate, and manganese sulfate with a molar ratioof nickel:cobalt:manganese=35:35:30, a 25% by mass aqueous sodiumhydroxide solution as an alkali solution, and a 25% by mass aqueousammonia solution as a complexing agent were simultaneously andcontinuously added, and a neutralization crystallization reaction wascarried out. The pH value and the ammonium ion concentration wereadjusted in such a way that the dissolved nickel concentration becameconstant at 1080 mg/L. At this time, the ammonium ion concentration inthe reaction vessel was in the range of 12 to 15 g/L. The total flowrate of the mixed solution, the aqueous sodium hydroxide solution, andthe aqueous ammonia solution was controlled in such a way that theresidence time of the metal salts included in the mixed aqueous solutionbecame 8 hours. The pH value at this time was 11.6 based on the liquidtemperature of 25° C. with the plus/minus fluctuation width of 0.1.After the reaction vessel became stable, the slurry including thenickel-cobalt-manganese composite hydroxide was recovered from anoverflowing port; and then, a cake of the nickel-cobalt-manganesecomposite hydroxide was obtained by suction filtration. Afterfiltration, impurities included therein were washed out by suctionfiltration with feeding 1 L of purified water to 140 g of the cake ofthe nickel-cobalt-manganese composite hydroxide present in thefiltration equipment. The cake of the nickel-cobalt-manganese compositehydroxide after being washed was air-dried at 120° C. to obtain thenickel-cobalt-manganese composite hydroxide (hereinafter, this is alsoreferred to as “composite hydroxide”).

The particle size distribution of the obtained composite hydroxide wasmeasured using a laser diffraction scattering type particle sizedistribution measurement apparatus. Consequently, the average particlediameter MV was 10.1 μm, and [(D90−D10)/the average particle diameter]was 0.78. The pore volume was measured by a nitrogen adsorption method.Consequently, the pore volume was 0.013 cm³/g. The tap density wasmeasured using a tapping apparatus (KYT 3000 manufactured by SeishinEnterprise Co., Ltd.) and was calculated from a volume and a sampleweight after 500 times of tapping. Consequently, the tap density was2.12 g/cm³. The specific surface area was measured by a BET method bynitrogen adsorption. Consequently, the specific surface area was 5.8m²/g.

The surface and the cross section structure of the obtained compositehydroxide were observed with a scanning electron microscope (SEM). FIG.5A and FIG. 5B respectively show the surface (FIG. 5A) and the crosssection structure (FIG. 5B) of the obtained composite hydroxide. Fromthe observation result of the surface, it was confirmed that thesecondary particle that has a high sphericity and is composed of theplate-like primary particles was obtained. From the observation resultof the cross section, it was confirmed that the inside of the particlehad a very dense structure. In order to assess the degree ofsparsity/density, the particle cross section and the void area withinthe particle were obtained by using the image analysis software (WinRoof6.1.1); and then, the degree of sparsity/density was calculated from theequation [(void area within the particle)/(particle crosssection)×100](%). Twenty cross sections of the secondary particles thatwere at least 80% of the volume-average particle diameter (MV) werearbitrarily selected, and the degree of sparsity/density of each of thecross sections of the secondary particles was measured; and the averagevalue thereof (average degree of sparsity/density) was calculated to be1.8%.

The obtained composite hydroxide was dissolved with an inorganic acidand was subjected to chemical analysis by ICP emission spectrometry, andit was revealed that its composition was Ni:Co:Mn=0.35:0.35:0.30 andthat particles with a target composition were obtained. Table 1 listscharacteristics of the obtained composite hydroxide.

[Production of Positive Electrode Active Material]

The above composite hydroxide and lithium carbonate were weighed so asto give a Li/Me of 1.06 and were thoroughly mixed together using ashaker mixer (TURBULA Type T2C manufactured by Willy A. Bachofen (WAB))with strength to the extent that the skeleton of the precursor wasmaintained to obtain a lithium mixture (the mixing process).

This lithium mixture was inserted into a firing vessel made of magnesia,and using an enclosed electric furnace, the temperature was raised up to950° C. at a temperature rising rate of 2.77° C./min in the atmospherewith a flow rate of 12 L/min and was held for 10 hours, and the lithiummixture was subjected to furnace cooling to room temperature to obtain alithium-nickel-manganese composite oxide (hereinafter, this is alsoreferred to as “lithium-transition metal composite oxide”) (firingprocess).

A surface and a sectional structure of the obtained lithium-transitionmetal composite oxide were observed with a scanning electron microscope,and it was revealed that particles having good sphericity were obtainedsimilarly to the composite hydroxide. Similarly to the compositehydroxide, particle size distribution measurement was performed on theobtained positive electrode active material. It was revealed that theaverage particle diameter D50 was 9.6 μm and that [(D90−D10)/the averageparticle diameter] was 0.80. The oil absorption amount and the tapdensity were measured to be 15.6 cm^(3/100) g and 2.40 g/cm³,respectively.

A surface and a sectional structure of the obtained positive electrodeactive material were observed with a scanning electron microscope (SEM).FIGS. 6A′ and 6B′ show the surface (FIG. 6A′) and the sectionalstructure (FIG. 6B′) of the obtained positive electrode active material.It was revealed that secondary particle that has a high sphericity andis formed of plate-shaped primary particles, similar to the shape of thecomposite hydroxide, was obtained. The result of sectional observationrevealed that the inside of the particle had a very dense structure. Thedegree of sparsity/density was calculated similarly to the compositehydroxide from the result of sectional observation to be 1.0%.

The obtained positive electrode active material was dissolved with aninorganic acid and was subjected to chemical analysis by ICP emissionspectrometry, and it was revealed that its composition wasLi_(1.06)Ni_(0.35)Co_(0.35)Mn_(0.30)O₂ and that particles with a targetcomposition were obtained. Table 2 lists characteristics of the obtainedpositive electrode active material.

[Orientation Evaluation]

The orientation of the obtained active material secondary particle inthe radial direction was evaluated by EBSD (electron backscatterdiffraction method). In view of keeping the conductivity of the targetsample for measurement, when the target sample was set inside themeasurement apparatus, fixing to the target sample holder was made byusing a conductive paste (colloidal carbon paste).

With regard to the measurement instrument, a scanning electronmicroscope (SEM) equipped with a computer capable of analyzing thecrystal direction (ULTRA 55; manufactured by Carl Zeiss GmbH) was used.The acceleration voltage of an electron beam irradiated to the targetsample was about 15 kV with the current amount of about 20 nA.

The orientation information in the x-axis and the y-axis was obtained inthe strip of 2.5 μm×12.5 μm in the area to measure the crystal directionin the cross section of the target sample (plane to be measured), inwhich the measurement was made at 250,000 points in total.

In order to easily take the picture of the electron beam scattered by acamera that is installed in the SEM apparatus (Kikuchi beam), the targetsample (more specifically the plane to be measured, i.e., the crosssection) was tilted about 70° from a horizontal plane so as to irradiatethe scattered electron beam to the camera.

The crystal direction of the material obtained by EBSD changes dependingon the direction of the standard axis chosen by the observer. Usually,the crystal direction distribution diagram is represented, as thestandard axis, by any axis of the orthogonal coordinate axes composed ofan x-axis, a y-axis, and a z-axis. Hereinafter, the crystal directiondistribution diagrams with the standard axis of the x-axis, the y-axis,and the z-axis are respectively called IPF-X, IPF-Y, and IPF-Z. FIG.7(A) and FIG. 7(B) are the schematic drawings that express theobserver's viewpoints corresponding to each crystal directiondistribution diagram. As illustrated in FIG. 7(B), when the observationcross section is regarded as the plane of the paper, IPF-X is thecrystal direction in the horizontal direction on this plane as thestandard. IPF-Y is in the perpendicular direction on this plane as thestandard. On the other hand, IPF-Z is the crystal direction in thevertical direction to the observation cross section as the standard.

In the case of the positive electrode active material, it is consideredthat the crystal direction information obtained when the edge of thepositive electrode active material particle in which the lithium ion istransferred with the electrolyte solution is observed from the particlesurface and the crystal direction information of the path in which thelithium ion inside the particle is de-inserted and that is in the radialdirection from the center of the particle to the outside thereof areimportant. Therefore, when the orientation evaluation of the x-axisdirection was carried out with regard to the radial direction of theparticle, the analysis results of the IPF-X corresponding to the crystaldirection observed from these directions were used; and similarly, theanalysis results of the IPF-Y were used for the orientation in they-axis direction.

The scattered electron beam (Kikuchi beam) was observed with a camera,and the data of the Kikuchi pattern observed with the camera were sentto a computer, and then, the Kikuchi pattern was analyzed to determinethe crystal direction. For each measurement point of the determinedcrystal direction data of the target sample, the coordinates (x and y)and the Euler angles (ϕ1, Φ, and ϕ2) that indicate the crystal directionwere obtained.

The measurement points having the values of the Euler angles that wereobtained by the sample evaluation were distributed to each crystaldirection as the zone axis in accordance with the following conditions.

<001> axis: ϕ1=0°±30°, Φ=0°±30°, ϕ2=0°±30°

<100> axis: ϕ1=0°±30°, Φ=90°±30°, ϕ2=60°±30°

<110> axis: ϕ1=0°±30°, Φ=90°±30°, ϕ2=120°±30°

In accordance with the rule described above, each measurement point canbe determined as to in which crystal direction the point is included.

After the above distribution, the ratio of each crystal direction in theplane to be measured was calculated by the number of the measurementpoints distributed to the respective crystal directions. The resultsthereof are listed in Table 2.

When executing this process, the commercially available analysissoftware for EBSD (analysis software for EBSD: Project Manager-Tango,sold by Oxford Instruments, Inc.) was used.

[Production of Battery]

Mixed together were 52.5 mg of the obtained positive electrode activematerial, 15 mg of acetylene black, and 7.5 mg of apolytetrafluoroethylene resin (PTFE), the resultant mixture waspress-formed at a pressure of 100 MPa to a diameter of 11 mm and athickness of 100 μm to form a positive electrode (an electrode forevaluation) PE illustrated in FIG. 8 . The produced positive electrodePE was dried in a vacuum drier at 120° C. for 12 hours, and then usingthis positive electrode PE, a 2032 type coin battery CBA was produced ina glove box in an Ar atmosphere and the dew point of which wascontrolled to −80° C. For a negative electrode NE, lithium (Li) metalwith a diameter of 17 mm and a thickness of 1 mm was used. For anelectrolyte solution, a liquid mixture of an equivalent amount ofethylene carbonate (EC) and diethyl carbonate (DEC) with 1 M LiClO₄ as asupporting electrolyte (manufactured by Tomiyama Pure ChemicalIndustries, Ltd.) was used. For a separator SE, a polyethylene porousfilm with a film thickness of 25 μm was used. The coin battery has agasket GA and a wave washer WW, and the coin-type battery was assembledwith a positive electrode can PC and a negative electrode can NC.

An initial discharging capacity was determined as follows: the producedcoin-type battery CBA was allowed to stand for about 24 hours, wascharged to a cutoff voltage 4.3 V with a current density to the positiveelectrode PE of 0.1 mA/cm² after an open circuit voltage (OCV)stabilized, and was discharged to a cutoff voltage 3.0 V after aone-hour suspension; and the capacity at this time was taken as theinitial discharging capacity. For the measurement of the dischargingcapacity, a multi-channel voltage/current generator (R6741A manufacturedby Advantest Corporation) was used. For reaction resistance, thecoin-type battery CBA was adjusted to have a measurement temperature andcharged at a charge potential of 4.1 V, and then a resistance value wasmeasured by an AC impedance method. For the measurement, using afrequency response analyzer and a potentiogalvanostat (1255Bmanufactured by Solartron), a Nyquist plot illustrated in FIG. 9 wascreated, and fitting calculation was performed using an equivalentcircuit illustrated in FIG. 10 to calculate a value of positiveelectrode resistance (the reaction resistance). From the result ofcharging/discharging measurement, a discharge voltage was calculated,and from this value, the tap density, and the initial dischargingcapacity, volume energy density was calculated from the expressionVolume energy density (Wh/L)=average discharge voltage (V)×dischargingcapacity (A/kg)×tap density (kg/L). Table 2 lists measurement results ofthe initial charging and discharging capacities, the positive electroderesistance value, and the volume energy density of the obtained activematerial.

Example 2

The composite hydroxide and the positive electrode active material wereprepared in the same way as Example 1 except that the stirring power inthe crystallization process was adjusted at 5.8 kW/m³, and that the N₂flow rate and the pH value were adjusted in such a way that in thereaction vessel the dissolved nickel concentration became 970 mg/L andthe dissolved oxygen concentration became 4.5 mg/L. The characteristicsof the obtained composite hydroxide are listed in Table 1. FIG. 5C andFIG. 5D are the surface (FIG. 5C) and the cross section structure (FIG.5D) of the obtained composite hydroxide, respectively; and FIG. 6C′ andFIG. 6D′ are the surface (FIG. 6C′) and the cross section structure(FIG. 6D′) of the obtained positive electrode active material,respectively. Table 2 lists the characteristics and the evaluationresults of the electrochemical characteristics of the obtained positiveelectrode active material. These evaluations were carried out in thesame way as Example 1.

Example 3

[Preparation of Composite Hydroxide]

A prescribed amount of purified water was taken into a reaction vessel(60 L), and the stirring power was adjusted at 6.0 kW/m³. Next, withstirring, the temperature inside the reaction vessel (liquidtemperature) was set at 45° C. At this time, a nitrogen gas (N₂) was fedin such a way that the dissolved oxygen concentration in the solution inthe reaction vessel became 3.5 mg/L by adjusting the N₂ flow rate. Intothis reaction vessel, a 2.0 mol/L mixed aqueous solution dissolvingnickel sulfate, cobalt sulfate, and manganese sulfate with a molar ratioof nickel:cobalt:manganese=60:20:20, a 25% by mass aqueous sodiumhydroxide solution as an alkali solution, and a 25% by mass aqueousammonia solution as a complexing agent were simultaneously andcontinuously added, and a neutralization crystallization reaction wascarried out. The pH value and the ammonium ion concentration wereadjusted in such a way that the dissolved nickel concentration becameconstant at 720 mg/L. At this time, the ammonium ion concentration inthe reaction vessel was in the range of 12 to 15 g/L. The total flowrate of the mixed solution, the aqueous sodium hydroxide solution, andthe aqueous ammonia solution was controlled in such a way that theresidence time of the metal salts included in the mixed aqueous solutionbecame 8 hours. The pH value at this time was 11.7 based on the liquidtemperature of 25° C. with the plus/minus fluctuation width of 0.1.After the reaction vessel became stable, the slurry including thenickel-cobalt-manganese composite hydroxide was recovered from theoverflowing port; and then, a cake of the nickel-cobalt-manganesecomposite hydroxide was obtained by suction filtration. Afterfiltration, impurities included therein were washed out by suctionfiltration with feeding 1 L of purified water to 140 g of the cake ofthe nickel-cobalt-manganese composite hydroxide present in thefiltration equipment. The cake of the nickel-cobalt-manganese compositehydroxide after being washed was air-dried at 120° C. to obtain thenickel-cobalt-manganese composite hydroxide (hereinafter, this is alsoreferred to as “composite hydroxide”). The surface and the cross sectionstructure of the obtained composite hydroxide were observed with ascanning electron microscope (SEM). FIG. 5E and FIG. 5F are the surface(FIG. 5E) and the cross section structure (FIG. 5F) of the obtainedcomposite hydroxide, respectively.

After the obtained composite hydroxide was dissolved into an inorganicacid, the chemical analysis thereof was carried out with an ICP emissionspectrometry; and as a result, the composition thereof wasNi:Co:Mn=0.60:0.20:0.20, so that it was confirmed that the particlehaving the intended composition was able to be obtained. Thecharacteristics of the obtained composite hydroxide are listed in Table1.

[Preparation of Positive Electrode Active Material]

After the composite hydroxide and lithium carbonate were weighed so asto give the Li/Me ratio of 1.03, they were fully mixed to obtain alithium mixture by using a shaker mixer (TURBULA Type T2C; manufacturedby Willy A. Bachofen AG (WAB)) with applying a strength that the shapeand structure of the precursor were able to be almost retained (mixingprocess).

The lithium mixture thus obtained was inserted into a magnesia-madefiring vessel, and by using a sealed-type electric furnace, thetemperature thereof was raised in an air atmosphere with the flow ratethereof being 12 L/minute and with the temperature raising rate of 2.77°C./minute until 900° C., at which temperature the mixture was kept for10 hours; and then, it was cooled in the furnace to room temperature toobtain the lithium-nickel-manganese composite oxide (hereinafter, thisis also referred to as “lithium-transition metal composite oxide”)(firing process).

The surface and the cross section structure of the obtained positiveelectrode active material were observed with a scanning electronmicroscope (SEM). FIG. 6E′ and FIG. 6F′ are the surface (FIG. 6E′) andthe cross section structure (FIG. 6F′) of the obtained positiveelectrode active material, respectively. After the obtained positiveelectrode active material was dissolved into an inorganic acid, thechemical analysis thereof was carried out with an ICP emissionspectrometry; and as a result, the composition thereof wasLi_(1.03)Ni_(0.60)Co_(0.20)Mn_(0.20)O₂, so that it was confirmed thatthe particle having the intended composition was able to be obtained.The characteristics of the obtained composite hydroxide are listed inTable 2. Each evaluation was carried out in the same way as Example 1.

Comparative Example 1

The composite hydroxide and the positive electrode active material wereprepared in the same way as Example 1 except that the stirring power inthe crystallization process was adjusted at 5.5 kW/m³, and that the N₂flow rate and the pH value were adjusted in such a way that in thereaction aqueous solution the dissolved nickel concentration became 410mg/L and the dissolved oxygen concentration became 5.8 mg/L. Thecharacteristics of the obtained composite hydroxide are listed inTable 1. The characteristics and the evaluation results of theelectrochemical characteristics of the obtained positive electrodeactive material are listed in Table 2. These evaluations were carriedout in the same way as Example 1.

Comparative Example 2

The composite hydroxide and the positive electrode active material wereprepared in the same way as Example 1 except that the stirring power inthe crystallization process was adjusted at 5.2 kW/m³, and that an airinstead of N₂ is fed with the changed flow rate and the pH value wereadjusted in such a way that in the reaction aqueous solution thedissolved nickel concentration became 300 mg/L and the dissolved oxygenconcentration became 6.2 mg/L. The characteristics of the obtainedcomposite hydroxide are listed in Table 1. The characteristics and theevaluation results of the electrochemical characteristics of theobtained positive electrode active material are listed in Table 2. Theseevaluations were carried out in the same way as Example 1.

Comparative Example 3

A composite hydroxide and a positive electrode active material wereproduced similarly to Example 3 except that the stirring power in thecrystallization process was adjusted to 5.8 kW/m³ and that the N₂ flowrate and the pH value were adjusted so as to give a dissolved nickelconcentration of 350 mg/L and a dissolved oxygen concentration of 5.8mg/L in the reaction aqueous solution. Table 1 lists characteristics ofthe obtained composite hydroxide. Table 2 lists characteristics andelectrochemical characteristic evaluation results of the obtainedpositive electrode active material. The evaluations were performedsimilarly to those in Example 1.

TABLE 1 Compara. Compara. Compara. Example 1 Example 2 Example 3 Example1 Example 2 Example 3 Crystallization Dissolved Ni (mg/L) 1080 970 720410 300 350 condition concentration Dissolved oxygen (mg/L) 2.8 4.5 3.55.8 6.2 5.8 concentration Stirring power (kW/m³) 6.0 5.8 6.0 5.5 5.2 5.8Crystallization (° C.) 45 45 45 45 45 45 temperature pH — 11.6 11.7 11.712.0 12.1 11.8 Composite Composition x 0.35 0.35 0.60 0.35 0.35 0.60hydroxide Ni_(x)Mn_(y)M_(z)(OH)₂ y 0.30 0.30 0.20 0.30 0.30 0.20 (M =Co) z 0.35 0.35 0.20 0.35 0.35 0.20 (001) plane (°) 0.283 0.367 0.2980.428 0.487 0.336 half width Average particle (μm) 10.1 10.2 11.6 10.110.1 11.5 diameter MV Degree of (%) 1.8 4.1 2.3 19.0 24.8 16.5sparsity/density Pore volume (cm³/g) 0.013 0.021 0.015 0.056 0.061 0.062(D90 − D10)/MV — 0.78 0.82 1.02 0.85 0.91 1.03 Specific surface area(m²/g) 5.8 12.2 11.1 14.7 18.5 13.3 Tap density (g/cm³) 2.12 1.95 2.21.50 1.21 1.76

TABLE 2 Compara. Compara. Compara. Example 1 Example 2 Example 3 Example1 Example 2 Example 3 Production Li/Me Ratio — 1.06 1.06 1.03 1.06 1.061.03 condition Firing temperature (° C.) 950 950 900 950 950 900Lithium-metal Composition t 0.06 0.06 0.03 0.06 0.06 0.03 compositeoxide Li_(1+t)Ni_(x)Mn_(y)M_(z)O₂ x 0.35 0.35 0.60 0.35 0.35 0.60 (M =Co) y 0.30 0.30 0.20 0.30 0.30 0.20 z 0.35 0.35 0.20 0.35 0.35 0.20Average particle diameter MV (μm) 9.6 9.8 11.4 9.3 9.2 11.3 Degree ofsparsity/density (%) 1.0 1.9 1.2 21.1 27.9 13.2 Tap density (g/cm³) 2.402.32 2.67 1.94 1.59 2.20 Oil absorption amount (cm³/100 g) 15.6 17.716.1 26.9 33.3 20.3 (D90 − D10)/MV — 0.80 0.81 1.00 0.82 0.91 1.00I(003)/I(104) — 1.95 1.74 1.72 1.91 1.96 1.59 ab plane X-axis (%) 59.863.3 66.1 68.3 62.0 65.5 orientation direction rate Y-axis (%) 63.2 75.083.6 45.8 39.5 51.7 direction Orientation ∘ ∘ ∘ x x x Battery Initialcharging capacity (mAh/g) 175.2 175.8 195.2 176.8 177.5 196.0characteristics Initial discharging capacity (mAh/g) 158.8 161.0 177.7163.1 164.4 179.1 Volume-based energy density (Wh/L) 1460 1431 1817 12121001 1509 Reaction resistance (Ω) 2.65 2.40 2.32 2.16 1.98 2.15

(Evaluation Results)

In Examples 1 to 3, because the dissolved oxygen concentration, thedissolved nickel concentration, and the stirring power are adjusted inthe optimum respective values, the composite hydroxide having a highreactivity with Li and a specific half width area, which is theindicator of the dense particle, is obtained. In addition, the resultsof the pore volume and the average degree of sparsity/density, too,indicate that the composite hydroxide having high density andfillability is obtained. The positive electrode active materialsynthesized from the composite hydroxide like this has, similarly to thecomposite hydroxide, high density and particle fillability, so that thevolume energy density thereof is high. In addition, in Examples 1 to 3,in the EBSD-based orientation evaluation, regarding orientations in theradial direction of both the x-axis and y-axis, the orientation rates ofthe ab plane, which is advantageous in insertion and de-insertion of Liions, were at least 55%, thereby indicating that the radial structurewas formed. In Example 1, from the SEM image of the cross section, too,in the region R2 that is 50% of the radius from the outer circumferenceof the secondary particle (positive electrode active material) to thecenter of the particle (see FIG. 3(B)), 60% of the primary particles innumber relative to the total number of the primary particles that werepresent in the region R2 were radially disposed from the center of thesecondary particle to the outer circumference thereof (radial directionR1, see FIG. 3(B)). Similarly, in Example 2 and Example 3, the primaryparticles of 83% and 87%, respectively, were radially disposed.

On the other hand, in Comparative Examples 1 to 3, because the dissolvedoxygen concentrations were higher than the conditions of Examples, thehalf widths of the composite hydroxides thereof were larger than thoseof Examples, so that the particles having high pore volumes and averagesparse densities were resulted. Accordingly, the particle fillabilitiesthereof were inferior to those of Examples. In the positive electrodeactive materials synthesized from these composite hydroxides, the volumeenergy densities were lower than Examples. In addition, in ComparativeExamples 1 to 3, in the orientation evaluation by EBSD, the orientationrates in the crystal ab plane in the y-axis direction were less than55%.

Accordingly, when the dissolved nickel concentration, the dissolvedoxygen concentration, and the stirring power are adjusted at the optimumrespective values, the composite hydroxide having high density andfillability can be obtained. In addition, when the nickel-manganesecomposite hydroxide like this is used, the positive electrode activematerial having a very high volume energy density can be obtained.

The technical scope of the present invention is not limited to theaspects described in the embodiment and the like. One or more of therequirements described in the embodiment and the like may be omitted.The requirements described in the embodiment and the like can becombined as appropriate. Japanese Patent Application No. 2016-150505 andall the literature cited in this specification are herein incorporatedby reference in their entirety to the extent allowed by law.

DESCRIPTION OF REFERENCE SIGNS

-   -   1 Nickel-manganese composite hydroxide    -   2 Primary particle (nickel-manganese composite hydroxide)    -   3 Secondary particle (nickel-manganese composite hydroxide)    -   4 Void (nickel-manganese composite hydroxide)    -   d Particle diameter of secondary particle    -   10 Positive electrode active material    -   11 Lithium-nickel-manganese composite oxide    -   12 Primary particle (lithium-nickel-manganese composite oxide)    -   13 Secondary particle (lithium-nickel-manganese composite oxide)    -   14 Void (lithium-nickel-manganese composite oxide)    -   C Central part of secondary particle (lithium-nickel-manganese        composite oxide)    -   L Direction of long diameter of primary particle    -   R1 Radial direction    -   R2 Area within 50% of a radius of the secondary particle from        the outer circumference toward the particle center

1. A method for producing a nickel-manganese composite hydroxiderepresented by General Formula (1): Ni_(x)Mn_(y)M_(z)(OH)_(2+α) (inFormula (1), M is at least one element selected from Co, Ti, V, Cr, Zr,Nb, Mo, Hf, Ta, Fe, and W; and x, y, z, and a satisfy 0.1≤x≤0.9,0.05≤y≤0.8, 0≤z≤0.8, x+y+z=1.0, and 0≤α≤0.4) and containing a secondaryparticle formed of a plurality of flocculated primary particles; themethod comprising a crystallization process of forming thenickel-manganese composite hydroxide by neutralizing a salt containingat least nickel and a salt containing at least manganese in a reactionaqueous solution; and in the reaction aqueous solution in thecrystallization process, a dissolved oxygen concentration is adjusted tofall within a range of at least 0.2 mg/L and up to 4.6 mg/L, and adissolved nickel concentration is adjusted to fall within a range of atleast 700 mg/L and up to 1,500 mg/L.
 2. The method for producing anickel-manganese composite hydroxide according to claim 1, wherein astirring power loaded to the reaction aqueous solution in thecrystallization process is adjusted to fall within a range of at least 3kW/m³ and up to 15 kW/m³.
 3. The method for producing a nickel-manganesecomposite hydroxide according to claim 1, wherein a temperature of thereaction aqueous solution in the crystallization process is adjusted tofall within a range of at least 35° C. and up to 60° C.
 4. The methodfor producing a nickel-manganese composite hydroxide according to claim1, wherein a pH value in the crystallization process measured based on atemperature of the reaction aqueous solution at 25° C. is adjusted tofall within a range of at least 10.0 and up to 13.0.
 5. The method forproducing a nickel-manganese composite hydroxide according to claim 1,wherein the crystallization process includes continuously adding a mixedaqueous solution including nickel and manganese into a reaction vesseland overflowing slurry including nickel-manganese composite hydroxideparticles formed by neutralization to recover the secondary particle. 6.A method for producing a positive electrode active material for anonaqueous electrolyte secondary battery, the positive electrode activematerial comprising: a lithium-nickel-manganese composite oxiderepresented by General Formula (2): Li_(1+t)Ni_(x)Mn_(y)M_(z)O_(2+β) (inFormula (2), M is at least one additional element selected from Co, Ti,V, Cr, Zr, Nb, Mo, Hf, Ta, Fe, and W; and t, x, y, z, and β satisfy−0.05≤t≤0.5, 0.1≤x≤0.9, 0.05≤y≤0.8, 0≤z≤0.8, x+y+z=1.0, and 0≤β≤0.5) andcontains a secondary particle formed of a plurality of flocculatedprimary particles, the method comprising: a process of providing anickel-manganese composite hydroxide represented by General Formula (1):Ni_(x)Mn_(y)M_(z)(OH)_(2+α) (in Formula (1), M is at least one elementselected from Co, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, Fe, and W; and x, y, z,and a satisfy 0.1≤x≤0.9, 0.05≤y≤0.8, 0≤z≤0.8, x+y+z=1.0, and 0≤α≤0.4)and containing a secondary particle formed of a plurality of flocculatedprimary particles, wherein the nickel-manganese composite hydroxide hasa half width of a diffraction peak of a (001) plane obtained by X-raydiffraction measurement of at least 0.10° and up to 0.40° and has adegree of sparsity/density represented by [(void area within secondaryparticle/cross section of secondary particle)×100](%) of at least 0.5%and up to 10%; a process of mixing the nickel-manganese compositehydroxide with a lithium compound to obtain a mixture; and a process offiring the mixture to obtain the lithium-nickel-manganese compositeoxide.
 7. The method for producing a positive electrode active materialfor a nonaqueous electrolyte secondary battery according to claim 6,wherein the nickel-manganese composite hydroxide is obtained by themethod comprising a crystallization process of forming thenickel-manganese composite hydroxide by neutralizing a salt containingat least nickel and a salt containing at least manganese in a reactionaqueous solution; and in the reaction aqueous solution in thecrystallization process, a dissolved oxygen concentration is adjusted tofall within a range of at least 0.2 mg/L and up to 4.6 mg/L, and adissolved nickel concentration is adjusted to fall within a range of atleast 700 mg/L and up to 1,500 mg/L.